It seems Valve have hardly paused for thought before creating a sequel to last year’s incredibly successful co-op zombie shooter Left 4 Dead. And why ever not? It was immediately lapped up by gamers and has gone on to be one of the most popular co-op gaming experiences ever made.

Left 4 Dead 2, it seems, will continue on very much in the same vein, forcing players to put aside their differences and continue the struggle against zombie apocalypse together in a set of new locations with a selection of new features to boot.

The sequel features four new Survivors; Rochelle, a news reporter, Coach, who’s unsurprisingly a school coach, Ellis, a mechanic, and Nick, a gambler chock full of cynicism and scorn. They’re unfortunate enough to be stuck amongst the undead in South America, working their way from Savannah towards New Orleans where the military are apparently (hopefully) waiting by to evacuate the uninfected.

New levels are set to include moments of daylight, which will be perfect to showcase just how far we’ll be able to knock limbs flying as L4D2 brings about the ability to knock off bits of bodies.

Valve have been busy at work refining the formula that made up the first title, listening carefully to the oodles of feedback from fans and perhaps more importantly: watching how they’ve been playing. Indeed, believe us when we tell you that most of the sneaky tactics you learned playing the first game are now completely defunct.

The AI Director has been vastly improved and given much more power to alter the content of the maps. It’s capable of changing the placement of walls, the lighting, weather conditions and even the placement of fences and other such path-altering objects. It’s programmed to often throw players into gauntlet situations where they’ll be forced down narrow corridors packed full of enemies.

One example Valve used to demonstrate the improved Director was a level in which the Survivors must make their way through a cemetery. In said cemetery the tombs are placed around the area in completely different layouts with every play according to how well, or how badly they’re doing. If the Survivors are performing particularly well then the Director will make progress through the graveyard a much longer and more difficult process by creating (un)dead ends or other such awkward pathways.

Valve have also created a bunch of both new and improved weapons for the sequel, including a couple of rather silly melee items: the frying pan and the axe, though more are apparently to come. These are powerful but temporary tools that boast a one-hit-kill to Common Infected and are incredibly helpful when backed into a corner by a swarm of brain-thirsty lumberers.

The submachine gun is back, as is the pump shotgun, assault rifle (though renamed to M-16 Assault Rifle), hunting rifle and auto shotgun (renamed to tactical shotgun). New to the sequel though is the chrome shotgun, the combat shotgun, the combat rifle, the AK-47 and the sniper rifle. Whether there might be more added to the list or not we don’t yet know.

Incendiary ammo has been added to the mix with some devastating results. The fire it creates is certainly something you’ll be wanting to keep a distance from but it’s fantastic for disposing with large crowds as it will spread violently from one zombie to another.

New classes are-a-coming to L4D2, of which so far we’ve seen the Charger, which is a pretty massive beast that picks a player to run at before attacking with some powerful force. The Witch is back and has been altered for the newly added instances of daylight, and in these times, instead of her usual tactic of sitting and crying she’ll wander through the maps making her much more difficult to avoid.

Read on for our interview with Valve’s Erik Johnson to find out more…

Prima: New Orleans is a cool setting, but it strikes me as a little bit risky - considering what that city suffered with Hurricane Katrina and its aftermath, with all the looting - setting a post-apocalyptic survival game there.

Erik Johnson: I think with things like that, it's kind of how you're dealing with it. But we chose New Orleans because of how cool of a city it is. We have people at Valve who grew up in New Orleans, we think the culture and the music is really cool, the architecture is beautiful. It makes a great setting for a spooky type of game.

To be honest, the Katrina thing wasn't... I think where you get into trouble is when you're being insensitive, when you're trying to use something like Katrina that happened and was really terrible as a way to present the game. Hopefully it doesn't come off as insensitive.

Prima: It seems quite soon to be bringing out a sequel for Left 4 Dead. It was Valve's big game last year and it's going to be the next. Why did you choose to do that?

Erik Johnson: Well, you know, we've had lots of successful game and taken different tacks in terms of what we were going to go build. We spend a lot of effort on the game design side and the business side, it involves everything we do, trying to figure out the optimal way to deliver content to customers.

It felt like - co-op was a new thing for us, and we learned a lot, the most we've ever done about a type of game, shipping Left 4 Dead 1, and we wanted to build on that. And the team was really efficient at building that kind of product and getting it out.

Prima: Will you be extending the SDK support from the first game to include Left 4 Dead 2?

Erik Johnson: Yep, we love people making mods and putting their own spin on our technology, so as much as we can do on that, we will.

Prima: The Xbox 360 exclusivity - was that a business decision, an arrangement with Microsoft? Or was it down to your preference for the Xbox platform?

Erik Johnson: We don't have a ton of experience internally building products for other console platforms. Left 4 Dead 1 was PC and Xbox, so it made a lot of sense to go for Xbox this time.

Prima: Valve is known for its continuing support of its PC games, the way you've handled Team Fortress 2, the way you support games through Steam... how have you found supporting an online multiplayer game like Left 4 Dead through Xbox Live? It must be a different experience.

Erik Johnson: It's harder. Absolutely it's harder. In the same way that we put a lot of effort into making internal development efficient, we want as much as efficiency as possible from people building games to the customers experiencing it. On the PC that's much easier of course, on the 360 it's just not.

Prima: Because you're in control of the update process on PC?

Erik Johnson: Yeah, and not just the update process but kind of everything. We know that every PC has a hard drive for example. We can't guarantee that on 360. We can allow ourselves to take iterative game design risks, because we know that if it doesn't work in the wild - the things that are difficult to test internally that require scale - that we can ship that to customers, say in Team Fortress 2, and if it doesn't work out we can change it back pretty easily. Customers kind of enjoy being part of that process. We're not afraid to make mistakes.

Prima: Are you going to continue to support the first game with more DLC and patches, or is the emphasis now on Left 4 Dead 2?

Erik Johnson: We're going to keep building stuff for Left 4 Dead 1. We'll support both. We've always felt that it makes good business sense to keep customers happy. It'll be good for Left 4 Dead 2 to support Left 4 Dead 1. We do have risks in that we don't want to split the two camps, we don't want a Left 4 Dead 1 group and a Left 4 Dead 2 group, that's a problem that we need to figure out.